Karbon Nanofiber İlave Edilmiş Mezofaz Zift Bazlı Karbon Köpüğün Hazırlanması Ve Karakterizasyonu

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ISTANBUL TECHNICAL UNIVERSITY  GRADUATE SCHOOL OF SCIENCE ENGINEERING AND TECHNOLOGY

Ph.D. THESIS

JUNE 2012

PREPARATION AND CHARACTERIZATION OF CARBON NANOFIBER ADDED MESOPHASE PITCH BASED CARBON FOAM

Ayşenur GÜL

Department of Advanced Techonologies Material Science and Engineering Programme

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JUNE 2012

ISTANBUL TECHNICAL UNIVERSITY  GRADUATE SCHOOL OF SCIENCE ENGINEERING AND TECHNOLOGY

Ph.D. THESIS Ayşenur GÜL (521042012)

Department of Advanced Techonologies Material Science and Engineering Programme

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HAZİRAN 2012

İSTANBUL TEKNİK ÜNİVERSİTESİ  FEN BİLİMLERİ ENSTİTÜSÜ

KARBON NANOFİBER İLAVE EDİLMİŞ MEZOFAZ ZİFT BAZLI KARBON KÖPÜĞÜN HAZIRLANMASI VE KARAKTERİZASYONU

DOKTORA TEZİ Ayşenur GÜL

(521042012)

İleri Teknolojiler Anabilim Dalı Malzeme Bilimi ve Mühendisliği Programı

Tez Danışmanı: Prof. Dr. M. Ferhat YARDIM

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vii FOREWORD

To endure my difficulties of my thesis, it takes guidance and encouragement from the following people.

First of all, I would like to express my special thanks and gratitude to my supervisor and mentor, Prof. Dr. M. Ferhat YARDIM with whom I worked for many years and who helped me in countless ways to bring me up in the scientific and academic area, and supervised me to carry out this study. I would like to express my deep and sincere gratitude to Prof. Dr. Mustafa ÜRGEN, Prof. Dr. A. Sezai SARAÇ for their continual encouragement, support, advices, and extensive knowledge throughout the study. Without their support this thesis would not have been accomplished.

I am also very thankful to Prof. Dr. Ekrem EKİNCİ, who pays attention to students’ academic growth, from which I benefit a lot, and for showing me understanding, giving me many valuable advices throughout my undergraduate and graduate studies. I would also like to send my special thanks to Prof. Dr. Yüksel AVCIBAŞI GÜVENİLİR and Prof. Dr. Tuncer ERCİYES for providing me both guidance and moral support during my post graduate studies at here ITU.

I am grateful to Turkish State Planning Organization (SPO) for financial support during my PhD.

This thesis was a product of long and intensive study which, with regard to experimental investigation, was the partly realized in Kyushu University in Japan. The long journey set out with the 6 month scholarship that was awarded by Istanbul Technical University and Kyushu University. I would like to express my special thanks and sincere appreciation to Prof. Isao MOCHIDA and Prof. Seong-Ho YOON, who gave me a chance to work in Materials Science Laboratories in Kyushu University.

I would like to thank to Prof. Dr Birgül TANTEKİN ERSOLMAZ, Prof. Dr. F. Seniha GÜNER, Prof. Dr. Gültekin GÖLLER, Prof. Dr. Hüseyin ÇİMENOĞLU and Prof. Dr. Nusret BULUTÇU, from ITU Chemical and Metallurgical Engineering Department for providing the scanning electron microscopy (SEM), He-pycnometry, X-ray diffraction (XRD) and mechanical strength analyses and their technical help and support.

I would like to thank Hüseyin SEZER, Mehmet OKUR (M.Sc.), Dr. Özgür ÇELİK who are conducted SEM, He-pycnometer and mechanical strength analyses.

I would also like to thank Işık YAVUZ (M.Sc.) and Esra ENGİN (M.Sc.) for their help with the SEM and XRD analyses.

I would like to thank to the Chemical Engineering and Metallurgical Engineering departments of Istanbul Technical University for their encouragement and support they have provided.

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I wish extend my thanks to many friends and colleagues who have been meaningful part of my life professionally and personally at ITU: Dr. Birgül ARSLANOĞLU, Dr. Yasin ARSLANOĞLU, Dr. İpek AKIN, Dr.Andelip AYDIN, Dr.Neslihan ALEMDAR, Dr. Hale DÜŞKÜNKORUR, Dr. Osman EKSİK, Dr.Çiğdem TAŞDELEN YÜCEDAĞ, Aylin KERTİK (M.Sc.), Gülçin UYGUR (M.Sc.), Taner BOSTANCI (M.Sc.), Esra IŞIKSAL (M.Sc.), Mesut KIRCA (M.Sc.), Pelin YAZGAN (M.Sc.), İlker DEMİRYOL (M.Sc.), Emre YÜZBAŞIOĞLU(M.Sc.). I would like to express my special thanks to my sisters (Zeynep, Necla, Jale, and Nigar) and brothers (Barbaros and Cengiz) who supported me patiently during this long journey and give this thesis as a gift to my mother, my father and my grandmother, who passed away several years ago, who could be proud of me for releasing this study.

June 2012 Ayşenur Gül, M.Sc (Chemical Engineer)

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ix TABLE OF CONTENTS Page FOREWORD………..………...vii TABLE OF CONTENTS………...ix ABREVIATIONS………...………...xi LIST OF TABLES………...………...xiii LIST OF FIGURES………...……...xv SUMMARY………..xix ÖZET………....xxi 1.INTRODUCTION………...………....1

2.GENERAL INFORMATION ABOUT CARBON AND CARBON FOAM...3

2.1 Carbon………...3

2.2 Bonding in Carbon Material………...…..4

2.3 Crystal Structures of Carbon………...……….….4

2.4 Historical Overview of Carbon Materials……….…..….6

2.5 Order and Disorder in Carbon Materials………..……...8

2.5.1 More ordered sructures………..………8

2.5.2 Less ordered structures………..………...9

2.6 Carbon Forms………..………...….9

2.6.1 Graphitic and non-graphitic carbons………...…..9

2.6.2 Graphitizable and non graphitizable carbons...10

2.7 Carbon Foam………...……….………....11

2.7.1 History of carbon foam………..………...11

2.7.2 Carbon foam precursors………..…….12

2.7.3 Properties of carbon foams………..…20

2.8 Preparation and Characteristic of Mesophase Pitch Derived Graphitized Carbon Foam...23

2.8.1 An overview of carbon foam technology………...…………...23

2.8.2 Carbonization of carbon foam………...………....26

2.8.3 Graphitization of carbon foam………...26

2.9 Carbon Nanofiber………..………..…………...27

2.10 Application areas of carbon foam………..…..31

2.10.1 Thermal management applications………...36

2.10.2 Energy storage………..………...38

2.10.3 Acoustic and electromagnetic absorption………..39

2.10.4 Batteries………..…...40

2.10.5 Composite tooling………..41

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3. EXPERIMENTAL PROCEDURE...45

3.1 Raw Material...46

3.1.1 Mesophase pitch...46

3.1.2 Carbon nanofiber production...48

3.1.2.1 Catalyst preparation...48

3.1.2.2 Herringbone carbon nanofiber production...49

3.2 Carbon Foam Production...50

3.2.1 Experimental setup...50

3.3 The process for the manufacture of carbon foam...52

3.4 Characterization of Carbon Foam...56

3.4.1. X-ray diffractometry...56

3.4.2. Scanning electron microscopy...57

3.4.3 Density...58

3.4.4 Helium pycnometry...58

3.4.5 Compressive strength...58

4. RESULTS AND DISCUSSIONS………...…...……....59

4.1Structure of Mesophase Pitch based Carbon Foams Produced at Low Pressures………...…...59

4.1.1 Properties of the carbon foam produced at 5 atm………...….59

4.1.2 Properties of the carbon foam produced at 10 atm...65

4.2 Effect of Carbon Nanofiber Additive on the Properties of Carbon Foam…….71

4.2.1 Properties of the carbon foam with the addition of CNF produced at 5 atm....71

4.2.2 Properties of the carbon foams with the addition of CNF produced at 10 atm……...97

5. CONCLUSIONS AND RECOMMEDATIONS ......131

5.1 Conclusions………...………..…...131

5.2 Recommendations and future work………...133

REFERENCES......135

APPENDICES...149

CURRICULUM VITAE………...………...157

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xi ABREVIATIONS

RVC : Reticulated vitreous carbon foam CNF : Carbon nanofiber

HCNF : Herringbone type carbon nanofiber SEM : Scanning Electron microscopy TEM :Transmission Electron microscopy XRD : X-Ray Diffraction

Lc : The crystal sizes in the c-direction La : The crystal sizes in the a-direction

d002 : The interlayer spacings between the closed-packed hexagonal planes U : Upper section

M : Middle section B : Bottom section

CF1 : Carbon foam with 1% (w/w) CNF additive CF3 : Carbon foam with 3% (w/w) CNF additive CF5 : Carbon foam with 5% (w/w) CNF additive CF7 : Carbon foam with 7 % (w/w) CNF additive CF10 : Carbon foam with 10% (w/w) CNF additive CF5-0 : Carbon foam produced at 5 atm (without additive)

CF5-1 : Carbon foam with 1% (w/w) CNF additive produced at 5 atm CF5-3 : Carbon foam with 3% (w/w) CNF additive produced at 5 atm CF5-5 : Carbon foam with 5% (w/w) CNF additive produced at 5 atm CF5-7 : Carbon foam with 7 % (w/w) CNF additive produced at 5 atm CF5-10 : Carbon foam with 10% (w/w) CNF additive produced at 5 atm CF10-0 : Carbon foam produced at 10 atm (without additive)

CF10-1 : Carbon foam with 1% (w/w) CNF additive produced at 10 atm CF10-3 : Carbon foam with 3% (w/w) CNF additive produced at 10 atm CF10-5 : Carbon foam with 5% (w/w) CNF additive produced at 10 atm CF10-7 : Carbon foam with 7% (w/w) CNF additive produced at 10 atm CF10-10 : Carbon foam with 10% (w/w) CNF additive produced at 10 atm

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xiii LIST OF TABLES

Page

Table 2.1 : Properties of several kinds of carbon foams……….22

Table 3.1 : Typical properties of mesophase pitch...47

Table 4.1 : Properties of carbon foam produced at 5 atm ………..64

Table 4.2 : Properties of carbon foam produced at 10 atm………...70

Table 4.3 : Comparison of properties of carbon foams produced at 5 and 10 atm………...……….71

Table 4.4 : Properties of the carbon foam with 1% CNF additive produced at 5 atm...78

Table 4.6 : Properties of carbon foam with 5 % CNF additive produced at 5 atm...87

Table 4.7 : Properties of carbon foam with 7 % CNF additive produced at 5 atm...91

Table 4.8 : Properties of the carbon foam with 10 % CNF additive produced at 5 atm...96

Table 4.9 : Properties of carbon foam with 1% CNF additive produced at 10 atm...102

Table 4.13 : Properties of the carbon foam with 10 % CNF additive produced at 10 atm...120

Tableı4.14 : Comparison of the samples with the addition of CNF at 5 and 10 atm...122

Table 4.15 : Comparison of the carbon foam with CNF additives produced at 10 atm...125

Table 4.16 : Comparison between CF10-0 and CF10-3......129

Table B.1 : The percentage of changes in some properties of produced carbon foams, at 10 atm………..………...…...151

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xv LIST OF FIGURES

Page

Figure 2.1 : Energy levels graph of carbon atom………....………...…...4

Figure 2.2 : Structure of diamond………..……...5

Figure 2.3 : Graphite structure………...…...5

Figure 2.4 : Fullerene Molecule………....6

Figure 2.5 : A diagram indicating the growth of carbon materials……...………7

Figure 2.6 : Marsh-Griffiths model of carbonization/graphitization process.......9

Figure 2.7 : Schematic representation of nongraphitizable (left) and graphitizable carbon (right)…………..………...……...10

Figure2.8 : A Typical RVC foam………...………...11

Figure 2.9 : Polyacrylonitrile derived carbon foam………….....13

Figure 2.10 : Mechanism of thermosetting foam preparation………...13

Figure 2.11 :a SEM images of carbon foam derived from different precursors (a) petroleum pitch (b) coal tar pitch………...15

Figure 2.12 : Coal-based carbon foam microstructure………...……....16

Figure 2.13 : Coal derived carbon foam tooling………...…….17

Figure 2.14 : Structure of mesophase pitch (a) AR mesophase pitch (b) A typical petroleum mesophase……….……….…....18

Figure 2.15 :a Block and process flow diagram of the naphthalene derived mesophase Pitch……….………...19

Figure 2.16 :a a) phenolic resole resin derived closed cell foam b) mesophase pitch derived open cell foam………...……...21

Figure 2.17 : Traditional “Blowing” technique ………...………...24

Figure 2.18 : Novel production method of ORNL………..………….....25

Figure 2.19 : Photomicrographs of high thermal conductivity graphite foam…....25

Figure 2.20 : The mechanism of graphitization………...……….......27

Figure 2.21 :a Carbon nanofiber structures according to the angle between fiber axis and graphitic layers ...28

Figure 2.22 : SEM and TEM pictures of platelet (a, b), herringbone (c, d) and tubular (e, f) ...29

Figure 2.23 :a Schematic representation of the growth mechanism of carbon nanofibers...30

Figure 2.24 : Critical factors in catalytic synthesis of CNF and their effects....30

Figure 2.25 : Machined carbon foam disk and machined joint………...………....31

Figure 2.26 : Carbon foam composite panel………...………...32

Figure 2.27 : High thermal conductivity foam-core composite with aluminum face sheets………...……….33

Figure 2.28 : Lightweight space mirrors support being machined on Touchstone's 5 ft by 9 ft three-dimensional router…….…………..33

Figure 2.29 : Lamination with reinforced vinyl ester face sheets…………...…..34

Figure 2.30 : Foam heat sink in Pentium 133 microprocessor………...………...37

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Figure 3.1 : Flow diagram of the process for making carbon foam...45

Figure 3.2 : Mesophase Pitch....46

Figure 3.3 : Heating regime during CNF synthesis process....49

Figure 3.4 : Aluminum mold......50

Figure 3.5 : Reactor.......50

Figure 3.6 : Computer software menu to control reactor temperature and pressure...51

Figure 3.7 : Carbonization furnace....52

Figure 3.8 : Schematic illustration of bubble growth process...53

Figure 3.9 : Bubbles growth under applied pressure....54

Figure 3.10 : The terminology used for SEM photographs of carbon foam....55

Figure 3.11 : Scanning Electron Microscope....57

Figure 3.12 : Compressive Strength Test Equipment....58

Figure 4.1 : A cylindrical shape of carbon foam sample………..…...59

Figure 4.2 : SEM images of upper, middle, and bottom sections of carbon foam produced at 5 atm from z-axis direction………...61

Figure 4.3 : SEM images of upper, middle, and bottom sections of carbon foam produced at 5 atm from x-axis direction……….……...62

Figure 4.4 : X-ray diffraction patterns of carbon foam produced at 5 atm....63

Figure 4.5 : SEM images of upper, middle, and bottom sections of carbon foam produced at 10 atm from z-axis direction………….………...67

Figure 4.6 : SEM images of upper, middle, and bottom sections of carbon foam produced at 10 atm from x-axis direction………..…...68

Figure 4.7 : X-ray diffraction patterns of the carbon foam produced at 10 atm………...69

Figure 4.8 : SEM images of upper, middle, and bottom sections for the carbon foam with 1% (w/w) CNF additive produced at 5 atm in the z-direction...73

Figure 4.9 : SEM images of upper, middle, and bottom sections for the carbon foam with 1% (w/w) CNF additive produced at 5 atm in the x- direction…...75

Figure 4.10 : XRD analysis of the carbon foam with 1% CNFadditive produced at 5 atm ………..……...76

Figure 4.11 : SEM images of upper, middle, bottom sections for the carbon foam with 3% (w/w) CNF additive produced at 5 atm in the z-direction………...79

Figure 4.12 : SEM images of upper, middle, and bottom sections for the carbon foam with 3% (w/w) CNF additive produced at 5 atm in the x-direction………...……….………....81

Figure 4.13 : XRD analysis of the carbon foam with 3% CNF additive produced at 5 atm………..………...82

Figure 4.14 : SEM images of upper, middle, and bottom sections for the carbon foam with 5% (w/w) CNF additive produced at 5 atm in the z-direction……….………...….84

Figure 4.15 : SEM images of upper, middle, and bottom sections for the carbon foam with 5% (w/w) CNF additive produced at 5 atm in the x- direction…...……….……….………...85

Figure 4.16 : XRD analysis of the carbon foam with 5% CNF additive produced at 5 atm………..………...86

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Figure 4.17 : SEM images of upper, middle, and bottom sections for the carbon foam with 7% (w/w) CNF additive produced at

5 atm in the z-direction………...….88 Figure 4.18 : SEM images of upper, middle, and bottom sections for the

carbon foam with 7% (w/w) CNF additive produced at

5 atm in x-direction……….………...89

Figure 4.19 : XRD analysis of the carbon foam with 7% CNF additive

produced at 5 atm……….90

5 atm in the z-direction...93 Figure 4.21 : SEM images of upper, middle, and bottom sections for the

5 atm in the x-direction……….………..94 Figure 4.22 : XRD analysis of the carbon foam with 10% CNF additive

produced at 5 atm……….….…..….95 Figure 4.23 : SEM images of upper, middle, and bottom sections for the

10 atm in the z-direction………...…………....……98 Figure 4.24 : SEM images of upper, middle, and bottom sections for the

10 atm in the x- direction……….…….……….……..99

Figure 4.25 : XRD analysis of the carbon foam with 1 % CNF additive

produced at 10 atm……..………..….…….100

10 atm in the z-direction………...………....103

10 atm in the x-direction……….………...…...104

produced at 10 atm………..………...105 Figure 4.29 : SEM images of upper, middle, and bottom sections for the

10 atm in the z-direction………..…108 Figure 4.30 : SEM images of upper, middle, and bottom sections for the

10 atm in the x-direction………...109

produced at 10 atm……….………...110

10 atm in the z-direction……….………..…...112 Figure 4.33 : SEM images of upper, middle, and bottom sections for the

10 atm in the x-direction……….………...113 Figure 4.34 : XRD analysis of the carbon foam with 7% CNF additive

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10 atm in the z-direction………..……...………….………..117

Figure 4.36 : SEM images of upper, middle, and bottom sections for the carbon foam with 10% (w/w) CNF additive produced at 10 atm in the x-direction………...………...………...118

Figure 4.37 : XRD analysis of the carbon foam with 10% CNF additive

produced at 10 atm………..………...119 Figure 4.38 : XRD patterns of the carbon foams with the addition of CNF

produced at 10 atm………..…………..126 Figure 4.39 : XRD patterns of the carbon foams without additive and with the

addition of 3% CNF produced at 10 atm…………...……...…....130 Figure A.1 : TEM images of HCNF………...…...…....150 Figure A.2 : SEM images of HCNF………...…....150 Figure C.1 : Carbon nanofibers located in foam junction area……..……..152

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SUMMARY

The recent development of new technology devices in industrial, commercial and military fields emerge the requirement of new material systems with the unique multi-functional characteristics. For instance, in thermal management applications such as heat exchangers, thermal conductivity is a critical property while strength and stiffness are also important. In biomedical orthopedic devices, the stiffness is critical and should be tailorable as the bone fracture heals. Carbon and graphite foams provide unique properties that meet these requirements and act as unique solutions for novel technologies.

Carbon foams are rigid, porous materials consisting of an interconnected network of ligaments with certain features such as low density (0.04-0.8 g/cm3), high temperature tolerance (up to 3000 oC in inert atmosphere), high compressive strength (up to 20 MPa), large external surface area with interconnected structure and adjustable thermal and electrical properties. The potential applications of carbon foam include such diverse areas a high-temperature thermal insulation, high thermally conductive heat sinks, electrodes for energy storage, energy absorption material, catalyst support and filters, etc.

The first carbon foams were developed by Walter Ford in the 1960s as reticulated vitreous carbon foams by carbonizing thermosetting polymer foams. RVC foam is a low cost material system for thermal insulation, impact absorption, porous electrodes, filtration and scaffolding. A few decades later in the 1990’s, the Air Force Research Laboratory (AFRL) discovered a new processing technique (thermodynamic flash) utilizing a mesophase pitch precursor and ignited a new wave of carbon foam research. More recently, Scientists at Oak Ridge National Laboratory developed an alternative process to manufacture graphitic carbon foams with good bulk thermal and electrical conductivity.

This dissertation is focused on interpreting the foaming mechanism, analyzing the preparation process of mesophase pitch based carbon foams with the addition of carbon nanofiber (CNF) and characterization of the properties of final products. In order to produce carbon foams, mesophase pitch was introduced into a cylindrical aluminum mold and then mold was placed in a stainless steel high temperature and pressure reactor. The reactor was purged with nitrogen to provide an inert atmosphere. Then pressure was applied and kept constant during heating. The samples were heated to 350oC with a heating rate of 5oC/min and after system was set at 350 oC for 2 hours, then heating was continued with a heating rate of 5oC/min until 600oC and again 30 min soak time was applied. Finally pressure was released rapidly and samples were cooled down to ambient temperature in order to obtain

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green carbon foam samples. The green carbon foams were carbonized by heating up to 1000 ºC (1 hour) under nitrogen in a horizontal furnace.

Carbon foam produced with this technique was characterized with scanning electron microscopy (SEM), X-ray diffractometry, helium pycnometry and compressive strength test equipment. Also densities of the samples were measured.

The structures and properties of the produced carbon foams were obtained with respect to the parameters involved and further using the characterization results. The effects of foaming pressure and carbon nanofiber additives on the structure were investigated.

As a result of these experiments it is found that; more homogenous, better structured, higher density, higher compressive strength and lower porosity carbon foams were derived at the pressure of 10 atm. The bulk and skeletal density of carbon foams exhibited a decreasing trend with increasing amount of additive. The compressive strength of the carbon foams reduced with the addition of carbon nanofiber.

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KARBON NANOFİBER İLAVE EDİLMİŞ MEZOFAZ ZİFT BAZLI KARBON KÖPÜĞÜN HAZIRLANMASI VE KARAKTERİZASYONU

ÖZET

Endüstriyel, ticari ve askeri alanlarda yeni teknoloji cihazlarinın gelişmesi ile özgün ve çok fonksiyonlu yeni malzeme sistemlerinin gereksinimi ortaya çıkmaktadır. Örneğin, ısı yönetimi uygulamalarında kullanılan ısı değiştiricilerde ısıl iletkenliğin yanı sıra güç ve sertlik de önemli olan özelliklerdir. Biyomedikal ortopedik cihazlarda sertlik özelliği kemik kırıklarının iyileşmesinde kritik bir parametredir ve ihtiyacı karşılayacak şekilde uyarlanabilmelidir. Karbon ve grafit köpükler, özgün özellikleri ile bu ihtiyaçları karşılayabilmektedirler. Yeni teknolojiler için özgün çözümler sağlamaktadırlar.

Köpük kelimesi genellikle gözenekli ve düşük yoğunluğa sahip malzemeler için kullanılır. Köpükleşme olayı gaz baloncuklarının, katı veya sıvı maddeler içinde dağılımıyla oluşur. Bazı malzemelerin köpüklerinin elde edilmesiyle o malzemelere pek çok alanda uygulama imkânı sağlamaktadır. Bu malzemelerden biri de karbon köpüktür.

Karbon köpükler gözenek yapılarına göre kapalı hücreli veya açık hücreli olmak üzere iki gruba ayrılırlar. Açık hücreli köpüklerde, köpük hücrelerinin sahip oldukları gözenekler diğer komşu hücrelerle bağlantılı durumdadır ve bu hücrelerde gaz giriş-çıkışı olmaktadır. Kapalı hücreli köpüklerde ise hücreler komşu hücrelerle aradaki gözenekler yoluyla ilişkili olmayıp yalıtılmış odacık durumundadır ve herhangi bir gaz taneciği giriş-çıkış yapamamaktadır. Genellikle kapalı hücreli köpükler, açık hücreli köpüklere göre daha yüksek dayanıma sahiptirler.

Karbon köpükler poliakrilonitril (PAN), poliüretan, polivinilklorür, fenolik polimer, petrol ve katran zifti, kömür, piroliz edilebilir organik bileşikler ve sentetik mezofaz zift gibi çeşitli başlatıcı malzemeler kullanılarak üretilebilirler. Karbon köpüğünün fiziksel özellikleri üretim yöntemlerine ve seçilen ana malzemeye göre değişiklikler göstermektedir. Basınç ve sıcaklık karbon köpük üretiminde önemli birer parametredir. Üretim aşaması sırasında değişik basınç ve sıcaklıklar uygulandığında köpüğün hücre yapısı, yoğunluk, dayanım, ısıl iletkenlik gibi özelliklerin de değiştiği gözlenmektedir.

Karbon köpükler birbiri ile bağlantılı ligament örgü yapısına sahip katılardır. Aynı zamanda düşük yoğunluğa (0.04-0.8 g/cm3

), yüksek sıcaklığa karşı dirence (inert ortamda 3000 oC’ye kadar), yüksek dayanıma ( 20 MPa’a kadar), birbiriyle bağlantılı yüksek dış yüzey alanına ve uyarlanabilir ısıl ve elektriksel iletkenliğe sahiptirler. Karbon köpükler düşük sıcaklıklarda (1273 K’nin altında) üretildiğinde düşük ısıl iletkenliğe sahip iken yüksek sıcaklıklarda üretildiğinde (2773 K ve üzeri) yüksek ısıl iletkenliğe sahip olduğu görülmüştür.

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Karbon köpüklerin geniş uygulama alanları vardır. Yüksek sıcaklıklarda ısı yalıtımında, yüksek ısıl iletkenlikli ısı kuyularında, enerji depolamak için elektrotlarda, enerji absorlayıcılarında, katalizör yataklarında, filtrelerde, kompozit yapımında, düşük ağırlıklı aynalarda, roket nozüllerinde, optik bençlerde, uydu uygulamalarında,katalitik konvörtörlerde, akustik uygulamalarda, fren balatalarında yangına dayanıklı bloklarda, aşındırıcı aletlerde ve benzeri pek çok alanda kullanılabilirler.

İlk karbon köpüğü 1960’lı yıllarda Walter Ford tarafından üretilmiştir. Ford termoset polimerlerin karbonizasyonu ile retiküle karbon köpük (RVC) elde etmiştir. RVC köpük, ısı yalıtımında, darbenin emilmesinde, gözenekli elektrotlarda, filtrelerde ve iskele yapımında kullanılan düşük maliyetli bir malzemedir. 1990’lı yıllarda ise Hava kuvvetleri Araştırma Laboratuvarı (AFRL), başlangıç malzemesi olarak mezofaz zifti kullanarak karbon köpük araştırmalarında yeni bir akım (termodinamik flaş tekniğini) geliştirmiştir. Daha sonraki yıllarda ise Oak Ridge Ulusal Laboratuvarındaki (ORNL) bilim adamları, iyi termal ve elektriksel iletkenliğe sahip grafit karbon köpükler üretimi için alternatif bir proses geliştirmişlerdir.

Bu çalışmada hammadde olarak Mitsubishi AR zifti kullanılmıştır. Bu zift sentetik naftalin türevli olup %100 anizotropik mezofazdır. Mezofaz zifti ilk olarak 1965 yılında keşfedilmiştir. Ziftin katılaşmadan önceki durumunda oluşan küreler ve mozaikler nedeniyle mezofaz terimi kullanılmıştır. Anizotropik mezofaz ısıl çözülmeden sonra belirli aromatik hidrokarbonlara dönüşen ara bir üründür.

Mezofaz zift yüksek ölçüde düzenli bir anizotropik sıvı-kristal sistemidir ve küresel formdadır. Mezofaz ziftin de elde edildiği ana malzemeye göre çeşitli tipleri mevcuttur.

Petrol, kömür katranı veya sentetik malzemeler mezofaz ziftin üretilmesinde kullanılmaktadır. Mitsubishi AR zifti gibi sentetik malzemelerden türetilmiş mezofaz ziftler kömür katranı veya petrol ziftinden türetilmiş mezofaz ziftlerine oranla daha homojen molekül dağılımına sahiptirler.

Karbon köpüğü üretmek için genellikle kendiliğinden köpürme tekniği kullanılır. Mezofaz zift önce basınç altında eritilir ve böylece düşük molekül ağırlıklı bileşikler oluşur. Zift içersinde polikondenzasyon ve bozunma reaksiyonları gerçekleşir ve düşük molekül ağırlıklı bileşikler buharlaşır. Düşük molekül ağırlıklı bileşiklerin buharlaşmasıyla hücreler, ligamentler ve hücreler arasındaki bağlantı noktaları oluşmaya başlar. Tüm bu değişimler birbiriyle bağlı karbon köpük yapısının oluşmasıyla sonuçlanır. Viskozite, sıcaklık ve basınç bu oluşum sırasında önemli bir role sahiptir. Bu süreçte ve daha sonraki ısıtma aşamalarında fiziksel ve kimyasal değişimler gerçekleşir. Karbon köpüğün özellikleri üretimde kullanılan başlangıç malzemesine, proses şartlarına (sıcaklık ve basınç) ve ısıl işlem koşullarına (ısıtma hızı, bekleme süresi vb) bağlıdır. Bu nedenle de istenilen özelliklere sahip karbon köpüğün üretilebilmesi için mezofaz zift bazlı karbon köpük oluşum mekanizması incelenmektedir.

Pek çok araştırmacı karbon köpüklerin üretim mekanizması üzerine araştırmalar yapmıştır. Yapılan çalışmalarda köpük yapısına proses parametrelerinin etkisi, hücre geometrisi ve köpüğün malzeme özellikleri modellenmiştir. Fakat bu modeller daha çok ideal köpük yapısı üzerinedir. Bu modellerin doğruluğunun artması için köpükleşme prosesi sırasında kabarcık yapısı ve değişimleri açığa kavuşturulmalıdır.

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Bu nedenle köpükleşme mekanizmasını ve köpüğün özelliklerini kontrol etmek amacıyla mezofaz zift bazlı köpüğün oluşum sürecini daha iyi araştırmak gerekmektedir. Küresel olmayan kabarcıkların büyüme mekanizmasını incelemek ve kontrol etmek için parametrik araştırmalar yapılmıştır. Ayrıca köpükleşme sırasında küresel kabarcıkların hareketi ve büyümesini ele alan sayısal çalışmalar da mevcuttur. Sonuç olarak başlangıç viskozitesinin düşürülmesiyle kabarcık büyümesinin ve hareketinin arttığını gözlemlenmiştir. Tasarlanan bu modeller istenilen özelliklere sahip karbon köpüğü üretmek için yapılan çalışmalara ışık tutmuştur. Karbon köpük üretim sürecini ve bu süreçte oluşan kabarcık şeklinin büyümesini açığa kavuşturmak için çeşitli çalışmalar yapılmıştır. Mezofaz ziftin köpükleşmesi sırasında kabarcık büyümesini incelenmiş ve ilk oluşan kabarcıkların erimiş zift içersinde homojen olarak dağılmadığını tespit edilmiştir. Karbon köpük içersinde üst kısımdan alt kısımlara doğru yoğunluk farklılıklarının olduğu pek çok araştırmacı tarafından kanıtlanmıştır. Fakat köpüğün özelliklerinin araştırılmasıyla ilgili bu çalışmalar kısıtlıdır.

Karbon köpükler genellikle yüksek sıcaklık ve basınç altında üretilir. Dolayısıyla karbon köpük üretim prosesi boyunca enerji tüketimi fazladır. Bu nedenle de üretim prosesinin fizibilitesini arttırmak için enerji tüketimini kontrol altına almak gerekmektedir. Bu çalışmada düşük basınç altında ve çeşitli yoğunluklarda karbon köpükler üretilmiştir. Bu çalışmada üretilen düşük ağırlıklı karbon köpükler ileriki çalışmalarda pek çok uygulama alanı bulabileceklerdir.

Düşük yoğunluklu karbon köpükler anten sistemleri, radyatörler, yakıt hücreleri, filtreler, düşük ağırlıklı zırhlar, bağlantı panelleri ve diş implantları gibi pek çok hava-uzay ve endüstriyel alanında kullanım alanına sahiptirler.

Bu tezin amacı köpükleşme mekanizmasının yorumlanması, karbon nanofiber katkılı mezofaz zift bazlı karbon köpüklerin hazırlık aşamasının incelemesi ve elde edilen sonuç ürünlerin karakterizasyonudur. Karbon köpük üretimi için mezofaz zift silindirik alüminyum bir kalıbın içine koyularak yüksek sıcaklık ve basınç reaktörüne yerleştirilmiştir. Reaktör içersinde inert bir ortam sağlamak için sistem azot gazı ile süpürülmüştür. Sonra sisteme basınç uygulanmıştır ve basınç ısıtma işlemi boyunca sabit tutulmuştur. Numuneler 350 o

C’ye 5oC/dak ısıtma hızıyla ile ısıtılmış ve bu sıcaklıkta iki saat bekletilmiştir. Daha sonra sistem 600 o

C ‘ye 5oC/dak ısıtma hızıyla ısıtılmış ve bu sıcaklıkta 30 dakika bekletilmiştir. Reaktör içersindeki gaz hızlıca boşaltılmıştır ve sonra sistem soğumaya bırakılmıştır. Elde edilen karbon köpükler 1000 oC de azot ortamında 1 saat süre ile karbonize edilmiştir.

Bu teknik ile üretilen karbon köpükler taramalı elektron mikroskobu (SEM), X-ışını difraktometresi, helyum piknometresi ve basma dayanımı ölçüm cihazı ile karakterize edilmiştir. Ayrıca numunelerin yoğunluğu ölçülmüştir.

Üretilen karbon köpüklerin yapıları ve özellikleri ilgili parametreler ile ve karakterizasyon sonuçları kullanılarak elde edilmiştir. Proses basıncının ve karbon nanofiber katkısının karbon köpüğü yapısı üzerine etkisi araştırılmıştır.

Yapılan deneylerin sonucunda, 10 atm basınçta daha homojen, daha yoğun ve daha dayanıklı karbon köpükler elde edilmiştir. Ayrıca 10 atm basınçta elde edilen karbon köpükler daha az gözeneklidir. Karbon köpüklerin iskelet ve yığın yoğunluğu katkı maddesi ilavesi ile azalan bir eğilimi göstermiştir. Karbon köpüklerin dayanımı karbon nanofiber ilavesi ile azalmıştır.

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1 1. INTRODUCTION

Mesophase pitch-based carbon foams possess low bulk density, open cell structure, moisture insensitivity, high mechanical strength, high thermal stability and low coefficient of thermal expansion. Beside these properties thermal and electrical conductivity and porosity can be tailored. Depending on its tailored properties carbon foams can be used in high impact energy and acoustic absorption as well as electromagnetic shielding material. Carbon foams have excellent potential for various applications such as rocket nozzles, engine components, high thermally conductive heat sinks, electrodes for energy storage and catalyst supports, etc [1–7]. Generally, self–bubbling technique is used for the production of carbon foams. A mesophase pitch precursor is firstly melted under pressure and sequentially the low molecular weight compounds are evolved. Polycondensation and decomposition of the pitch is occurred and low molecular weight compounds begin to vaporize. The evaporation of light components initiates formation of cells, junctions and ligaments between the cells. All these changes end up with an interconnected foam structure. [8-10]. Viscosity, temperature, and pressure play an important role during this phenomenon. Physical and chemical changes take plays at this and in the subsequent further heat treatment stages. The properties of carbon foam depends precursor used in the manufacturing process, the process conditions (temperature and pressure) and the heat treatment. Thus, it is significant to further understand the formation mechanism of carbon foams derived from mesophase pitch, for adjustment of the properties of carbon foams [11, 12].

Substantial research has been carried out on growth mechanism of carbon foams by several researchers. Foam structure with processing parameters, microstructural geometry, and material properties of the foam are predicted in many model studied. However these models are much based on the ideal structures of the foam. The bubble structure and their changes during the foaming process must be completely understood in order to increase the accuracy of these models [12-17]. Beechem et al. predicted a growth mechanism of a non-spherical bubble assisted for a carbon foam

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fabrication and obtained a greater qualitative understanding of bubble shape during growth [15]. A parametric study highlighting the effect of the non-spherical growth of the bubble was performed in order to indicate how controllable bubble growth could be achieved. Rosebrock et al. carried out a numerical study to explore growth and movement of spherical bubbles during foaming [16]. It was found that decreasing the initial viscosity produced an increase in both the bubble growth and movement. The predicted results shed light on the carbon foam formation, which helped to achieve the adjustment of carbon foams. Klett et al. demonstrated that the aromatic units rearranged parallel to the axis of ligaments under a certain stress [12]. Wang et al. studied the bubble growth during mesophase pitch foaming. They observed that the initial foam bubbles were not uniformly dispersed in the molten pitch. The phenomenon of bulk density gradient of carbon foam was proved by some researcher [17- 19]. However, investigation related in sections from top to bottom of the foams is not discovered. Limited studies about addition of various additives including carbon nanofiber (CNF) can be found in the literature [20-27] but the effect of carbon nanofiber addition on density gradient is not well understood and beside this there are more rooms to understand the effect of CNF addition to final properties of carbon foams. Therefore, it is necessary to study further the evolution process of mesophase pitch derived carbon foams in order to control the foaming mechanism of pitch, and thus to adjust their properties.

Carbon foam production requires high energy consumption due to high pressure and high temperature in the process therefore it is very important to diminish the energy consumption for the feasibility of the process. In this study, carbon foams were obtained by low pressure with various density and further studies will reveal the application of area of these foams on the designated areas.

The application of carbon foams varies depending to the properties of the carbon foams. Light weight carbon foams are widely used in many industrial and aerospace applications such as antennae systems, radiators, fuel cell, filter, light armor, joiner panels, and tooth implants.

In this work, mesophase pitch derived carbon foams are studied by scanning electron microscopy (SEM), X-ray Diffractometry (XRD), He-pycnometry, and compressive strength measuring instrument. Effects of the carbon nanofiber addition on pitch including examination of final product properties are discussed in detail.

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2. GENERAL INFORMATION ABOUT CARBON AND CARBON FOAM

2.1 Carbon

Carbon is the most important element for all living organism on the Earth, because all organic compounds are composed from carbon networks. Carbon materials, which consist of mainly carbon atoms, have been used since prehistoric times in the form of charcoal [28].

Carbon has an atomic weight of 12 and it is the sixth element in the periodic table. Three isotopes are mostly known: 12C, 13C, 14C. Carbon-12 and carbon-13 are stable isotopes. They do not spontaneously change their structure and disintegrate.12C accounts for around 99% of the naturally occurring carbon and is used as reference definition of atomic mass. 13C is used as probe in nuclear magnetic resonance because of its magnetic moment (spin=1/2). 14C is radioactive and generated in the earth’s upper atmosphere by the interaction of neutrons with nitrogen. Also, it has very long half-life of 5730 years and is used extensively in the dating of archaeological artifacts and as a ‘label’ in the study of organic reaction mechanisms [28,29].

N14+n→C14+H1 (2.1) The properties of carbon based materials depend upon its electronic configuration (Figure 2.1). Carbon has four electrons in its valance shell (outer shell). Since the energy shell can hold eight electrons, each carbon atom can share electrons with up to four different atoms. Carbon displays"catenation" (bonding to itself) to such a degree that the number of resulting chains, rings, and networks are almost limitless. It makes advantages to combine with different kind of atoms [29].

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4 E N E R G Y 2s 2px 2py 2pz 1s

Figure 2.1 : Energy levels graph of carbon atom [30].

2.2 Bonding in Carbon Material

Bonding in carbon compounds is explained by two principle regimes as described below:

σ−bonds: diamond or aliphatic type. This result in chain of carbon atoms such as polyolefine’s, or three dimensional structures which are rigid and isotropic.

A mixture of σ and π bonds: graphite aromatic type. This results in predominantly layered structures with high degree of anisotropy.

The majority of carbonaceous materials contain examples of both bonding regimes with an immense range of complexity [29].

2.3 Crystal Structures of Carbon

Carbon atoms can have three different hybrid orbital, sp3, sp2, and sp. This variety makes carbon atom to have different forms therefore there are many kinds of carbon allotropes. Diamond, graphite and fullerene are mostly known. C-C bonds using sp3 and sp2 hybrid orbital was known in the construction of diamond and graphite respectively. Fullerene is constructed by combining sp and sp2 hybrid orbital [31]. Diamond is the one of the hardest material and has no color. Its structure consists of regular three dimensional networks of sp3 σ bonds with long range periodical repetition. Most diamond crystal (Figure 2.2) belongs to cubic system. Diamond is

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used as an electrical insulator because of its fixed bonding electron within the diamond lattice. It is used as industrial cutting tools due to its hardness [31,32].

Figure 2.2 : Structure of diamond [33].

Graphite structure (Figure 2.3) has sp2 σ and π bonds. It represents the hexagonal crystal system with the regularity of ABAB. There is a σ bond between the layers and the π bonds between the stack. The small amount of material is stacked according to the ABCABC which is known as rhombohedral form. This material accounts for less than 10% of the graphite. Graphite is a good conductor of heat and electricity. Since the energy for sliding layers over one another is low, graphite is very soft material. Thus, it used as a lubricant [29].

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Fullerene (Figure 2.4) was discovered in 1985. It is the cage molecule of carbon. The nature of bonding close to the sp2 bond but it isn’t clear. The hybridization is modification of the sp3 hybridization and sp2 hybridization. It means the sigma (σ) orbital don’t display σ character, and the pi (π) orbital don’t display π character purely. Fullerene is the one of the well known superconductive material [31]

Figure 2.4 : Fullerene Molecule [35].

2.4 Historical Overview of Carbon Materials

Carbon, in the form of charcoal, is an element of prehistoric discovery and was familiar to many ancient civilizations. A historical perspective of carbon and its allotropes are shown in Figure 2.5 [28].

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7 Pre-1880

Lamp Black (writing)

Charcoals (gunpowder, medicine, deodorants)

Natural graphite (writing material) Coal coking (coal-tar pitch)

Delayed coking

1940-1999

Carbon fibers (PAN) Carbon fibers (pitch-based) Carbon fibers (microporous) Carbon / resin composites

Carbon / carbon composites Speciality activated carbons Carbon as a catalyst support Carbon wiskers / filaments Prosthetics

Intercalation compounds Graphite / oxide refractories

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8 Pyrolytic carbon Glassy carbon Mesocarbon microbeads Diamond films Diamond-like films Elastic carbon Fullerenes Nanotubes Nanorods

High thermal conductivity graphite foams

Figure 2.5 (continued) : A diagram indicating the growth of carbon materials.

2.5 Order and Disorder in Carbon Materials

Carbon material can be classified with respect to the proportion of the ordered and disordered structure. The proportions of them are related with the properties of the material.

2.5.1 More ordered structures

Ordered structure can be imagining by graphite lattice. Small volumes exhibit perfect graphite crystal structure. As volume increases, the presence of defects, distortions and heteroatom destroy the regularity and produce disordered material. Layers can be slide over another layer by little amount of energy. Also twisting makes the structure parallel and equidistant layers, but with random orientation. These are called ‘turbostratic’ carbons [29,36].

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The diamonds like parts of the most carbon materials have shorter range order than graphitic regions, although large proportions of the disordered parts are aliphatic order. Defects and irregularities can be observed in the long range orders [29].

2.5.2 Less ordered structures

Carbon materials can be classified by the isotropy and anisotropy. Anisotropic carbons have ordered and graphitic structure. Isotropic carbons have randomly arranged materials. The order can be increased by further heat treatment. The crystal structure changing by heat treatment is shown in Figure 2.6 [29].

Figure 2.6 : Marsh-Griffiths model of carbonization/graphitization process [29].

2.6 Carbon Forms

2.6.1 Graphitic and non-graphitic carbons

Graphitic carbons are all varieties of material consisting of the element carbon in the allotropic form of graphite, irrespective of the presence of structural defects. Natural graphite is a mineral consisting of carbon regardless of crystalline perfection. Some natural graphite’s show a high degree of perfection but most are mined in the form of flake graphite’s containing other mineral matter [37].

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Synthetic graphite is defined as a material consisting mainly of graphitic carbon, which has been obtained by means of a graphitization heat treatment of a non graphitic carbon or by chemical vapor deposition (CVD) from hydrocarbons at temperatures above 1800ºC [37].

Non-graphitic carbons are all varieties of substances consisting mainly of the element carbon with two-dimensional long range order of the carbon atoms in planar hexagonal networks, but without any measurable crystallographic order in the c-direction. Many non-graphitic carbons can be converted to graphite by graphitization heat treatment to above 2200 ºC.

2.6.2 Graphitizable and non graphitizable carbons

Non-graphitizable carbons (Figure 2.7) can not be transformed into graphitic carbon solely by heat treatment at temperatures of 3000ºC or above under atmospheric or lower pressures. Non Graphitizable carbons are produced from wood, nutshells and non fusing coals. During heat treatment, their macromolecular structure does not change. Fusion can not take place only small molecules leave from the structures, and at the same time more cross linking structure occurs [37].

Graphitizable carbons can pass fluid stage during the heat treatment. Molecules can grow and large aromatic molecules can be formed. Thus they align with each other so graphitic structure can be developed [37].

Figure 2.7 : Schematic representation of nongraphitizable (left) and graphitizable carbon (right) [37].

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11 2.7 Carbon Foam

2.7.1 History of carbon foam

Walter Ford first developed carbon foam in the late 1960’s. This initial carbon foam was produced by carbonizing thermosetting polymer foams to obtain reticulated vitreous (glassy) carbon foam which is shown Figure 2.8. This initial carbon foam was obtained by the carbonization of foams of plastic materials, such as phenolic resins and polyurethanes [5, 38].

Figure 2.8 : A Typical RVC foam [12].

Googin et al. produced carbon foam by polymerization of furfuryl alcohol and urethane to get partially cured urethane foam in 1967. It was the first process to controlling the structure and material properties of carbon foam [39].

Researches focused on variety of applications of carbon foam. Carbon foam was used as electrode, insulator, filter, catalyst bed and catalyst support. In addition, carbon foam was used as the template for many of the metal. In 1970’s and 1980’s, alternative precursors and processing conditions were explored for producing carbon foam and modifying its properties [40-43].

In the 1970’s, researches focused primarily on producing carbon foams from alternative precursors, various processing and precursor changes in an attempt to modify properties and reduce cost.

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In 1976, Raley et. al. used vinylidine chloride polymers with ammonia to derive carbon foam. This carbon foam had been used as a catalyst support and filter material for gases, such as cigarette smoke and liquids [41].

In 1981, Bonzom et. al. developed carbon foam from petroleum and coal tar pitch. The carbon foam was used for thermal and sound insulating [1].

In 1988, Hopper dissolved pulverized sodium chloride particles and phenolic polymeric resin into tetrahydrofuran (THF) as a precursor of carbon foam [44]

Mesophase pitch derived carbon foam was discovered in the early 1990’s. This work was focused on the developing a highly structural lightweight material which exhibits very high specific thermal conductivity [2].

In 1997, Klett, J. at the Oak Ridge National Laboratory (ORNL) reported the first graphitic foams with bulk thermal conductivities up to 180 W/m.K. They used naphthalene derived mesophase pitch as a precursor. Due to high bulk thermal conductivity, the graphitic carbon foam is a potential material as thermal management materials [7, 8, 45,46].

2.7.2 Carbon foam precursors

Carbon foam has been produced from different kind of precursors since 1960’s such as polyacrylonitrile (PAN) [47-50], polyurethane [40,48], polyvinylchloride [51-54], phenolic polymer [55], coal tar [56,57] and petroleum pitch [1,58], coal [59], synthetic mesophase pitches [3, 51-55, 57,58,60,61].

• Polyacrylonitrile

Polyacrylonitrile in a heated alkali metal halide solution was used for producing carbon foam. PAN (polyacrylonitrile) was inserted in containment then sufficiently cooled the heated solution to form a liquid gel of the polyacrylonitrile by phase inversion. Removing the dissolved alkali metal halide from the gel provided porous foam consisting essentially of polyacrylonitrile. Cross-linking the polyacrylonitrile foam was obtained by removing residual traces of solvent under vacuum. Porous foam was oxidized at an elevated temperature in an oxygen-containing environment (e.g., air). Finally porous foam was heated in an inert atmosphere to a temperature sufficient to carbonize the polyacrylonitrile to provide a microcellular carbon foam product. Carbon foam produced with this method has uniform distribution of cell size

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and open porosity. It is used as catalyst support, absorbents, filters and electrodes [47]. A SEM micrograph of Polyacrylonitrile derived carbon foam is shown in Figure 2.9 [62].

Figure 2.9 : Polyacrylonitrile derived carbon foam. • Thermosetting polymer

"Glassy" or reticulated vitreous carbon (RVC) (Figure 2.8) foams are formed by curing and carbonization process of thermosetting resin-based precursors such as polyurethane, polyamide, polycarbodiimide, epoxy, phenol [40, 63-65]. Most thermosetting foams are prepared by simultaneous occurrence of polymer formation and gas generation. Principle of preparation of thermosetting foams is shown in Figure 2.10 [65].

Figure 2.10 : Mechanism of thermosetting foam preparation. Monomer

Blowing agent Catalyst Surfactant

(

Mixing

)

Polymer Formation

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They are used for many aerospace and industrial applications such as thermal insulation, impact absorption, acoustic control, catalyst support, and metal and gas filtration. Reticulated carbon foam is poor in oxidation resistance at high temperatures and corrosion resistance in a chemical reaction and is insufficient in thermal conductivity because of their amorphous carbon morphology [56, 60]. However they are thermally stable, low in weight and density, and chemically pure. They resist thermal stress and shock, and are relatively inexpensive [66].

Phenolic resin based carbon foam can be used for high-strength thermal insulation material and has been prepared by using various methods. Union Carbide and Ultramet have carried out studies on carbon foam since 1960. Union Carbide employed the methods of pyrolysis of phenolic foam and carbonization of phenolic microsphere mixture with phenolic resin to produce carbon foams with various pore sizes and densities (pore size of 25–150 µm at the density of 0.05–0.25 g/cm3) [38, 67-71]. Ultramet prepared the carbon foam by impregnating polyurethane foam template with phenolic resin then carbonizing [71-73]. Shiwen et.al tried to achieve phenolic-based carbon foams with controllable pore structure and high compressive strength. Average pore size of carbon foam was controlled by changing the resin concentration. Carbon foam with bulk density of 0.73 g/cm3, average pore size of 20 nm, compressive strength of 98.3 MPa and thermal conductivity of 0.24 W/mK was obtained [74].

• Coal tar or petroleum pitch

The coal tar or petroleum pitch based carbon foam is used as thermal insulation, as catalyst supports or as filters for corrosive products [64]. Although Mitsubishi mesophase AR pitch can be foamed directly without pretreatment, most coal and petroleum-derived pitches need to be treated before foaming can be achieved. Plastic properties of coal and petroleum-derived pitches are not enough for foaming. The properties and composition (viscosity and degree of anisotropy) of coal–tar pitches can be controlled by the temperature of thermo-oxidation treatment, amount of added acid and conditions of heat treatment. Commercially available pitches, such as Ashland A240 petroleum and Koppers coal tar pitch are not suitable for making carbon foam directly. Viscosity of these precursors is too low to hold the foam cell shape. Therefore, the pitch properties of these materials were tailored by heat treated in an autoclave between 200 oC and 400 oC under N2 atmosphere [11]. B. Tsyntsarski

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et.al are used H2SO4 and HNO3 as a chemical agent to modify the commercial coal–

tar pitch. Carbon foams with an anisotropic texture and high mechanical strength were produced using precursors obtained after thermo-oxidation treatment of commercial coal–tar pitch with H2SO4 and HNO3 [75]. Figure 2.11 shows the SEM

images of carbon foams derived from the petroleum pitch and coal tar pitch [11].

Figure 2.11 : SEM images of carbon foam derived from different precursors (a) petroleum pitch (b) coal tar pitch [11].

• Coal

The properties of coal vary widely, and thus some coals are suitable as foaming precursors and others are not. The foaming behavior of coal precursors is strongly related to their plastic properties, which are dependent on the maceral composition of the coal. Liptinite exhibits strong dilatation power but inertinite does not, while

a

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vitrinite is intermediate. Therefore, the selection of the appropriate coal precursor is important [11, 76]. The coal-based carbon foam with open cell and interconnected pores is shown in Figure 2.12 [77].

Figure 2.12 : Coal-based carbon foam microstructure.

While using a raw coal or hydrogenated coal, as a precursor, it is de-ashed and its asphaltene fraction is separated by a solvent treatment. The coking of the asphaltene fraction under controlled conditions of temperature and pressure results in the formation of a carbon foam, which can be subsequently graphitized [69]. The production method, developed by a research group at West Virginia University in USA, was licensed to Touchstone Research Group under the trade name CFOAM™ [77]. Touchstone Research Laboratory, Ltd. (Touchstone) has developed a tooling system using a coal-based carbon foam (CFOAM®) that obviates many of the concerns associated with alloy-based tools (Figure 2.13) [78].

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Figure 2.13 : Coal derived carbon foam tooling.

Coal based carbon foams have lightweight and their costs are relatively low. It can be used for efficient conductor of heat and electricity. Also, depending upon the end use, the degree of thermal conductivity can be altered. In general, the isotropic foam is not a high thermal conductor and the non-graphitized foam is poor thermal conductor. Coal based carbon foam have numerous usage area such as separator for ions in membranes, substrates for integrated circuits or aerospace applications, high temperature filters, a substitute for wood and steel beams in building and structural members, automotive parts ( pistons, vehicle frames, impact absorbers for doors and connecting rods exist ), aerospace and airplane parts ( wings, brakes, satellite and space station structure ) [59,79,80].

• Mesophase pitch

The mesophase phenomenon is discovered in 1965 by Brooks and Taylor [81]. Mesophase means sphere and mosaic substances that form before the solidification. When hydrocarbon is heated under inert atmosphere, it condenses to large planar molecules. As the molecules grow, they nucleate and grow a liquid crystal phase, called the mesophase. The liquid crystal phase consists of stacking planar molecules that are footprint of the graphitic pellets [54-55].

The mesophase pitch based carbon foam was produced at the Wright Patterson Air Force Base Materials Lab in 1990’s [2 ]. It is a synthetic naphthalene derived pitch, which is 100% anisotropic mesophase.

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Mesophase pitch is derived from various precursors such as petroleum and coal tar and other synthetic precursors. Mesophase pitches derived from synthetic precursors, such as Mitsubishi AR pitch which is prepared by the catalytic polymerization of naphthalene using HF-BF3 catalyst have more homogeneous compositions compared

to mesophase pitch derived from petroleum and coal tar pitch [82-84]. Figure 2.14 schematically shows the presentation of the structures of Mitsubishi AR and typical petroleum mesophase pitches [12].

(a) (b)

Figure 2.14 : Structure of mesophase pitch (a) AR mesophase pitch (b) A typical petroleum mesophase.

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Figure 2.15 shows a simplified bloc and process flow diagram of the naphthalene derived mesophase pitch.

Figure 2.15 : Block and process flow diagram of the naphthalene derived mesophase pitch [85].

Naphthalene and HF and BF3 are mixed in a stirred tank reactor to form the

naphthalene HF–BF3 complex. The AR complex section is where the naphthalene

complex is further reacted with naphthalene to form the AR/ HF–BF3 in a stirred

tank reactor. After the polymerization reaction, the AR/HF–heating in the stirred tank reactor decompose BF3.HF and BF3 are recovered and recycled to the naphthalene

complex section. At the same time, the light oil is recovered to assure 100 % anisotropy [85].

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The purification section is where nitrogen is blown into the purification tank to remove the low-boiling products, so adjusting the volatile contaminants in the AR resin. Solid contaminants are removed by filtration. Very pure AR resin melt is extruded into strands and cut into pellets [85].

Synthetic mesophase pitch is the preferred precursor for high thermal conductivity carbon materials because it is graphitizable [3,45] . When a synthetic mesophase pitch is used, the domains are stretched along the cell walls of the foam structure and thereby produce a highly aligned graphitic structure parallel to the cell walls [3]. This implies that the internal structure of a carbon materials produced from mesophase pitch is similar to that of graphite [45].

2.7.3 Properties of carbon foams

Carbon foam is complex cellular structures. The word cell is derived from the latin word "cella," which is a small compartment. A cluster of cells is a cellular solid. Honeycomb materials would be the simplest of cellular structures. They used for industrial, architectural and transportation application such as sandwich panels, molded parts, wind tunnels etc. A cellular solid consists of an interconnecting network of struts or plates forming the edges and faces of the cells. Foam consists principally of packed polyhedral structures, which can either be open or closed cell [81]. If the pores of the foam are connected to one another, the foam is defined as open cell foams. If the pores of the foam are isolated within the mass, the foam is defined as closed cell foams [86] (Figure 2.16).

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Figure 2.16 : a) phenolic resole resin derived closed cell foam [87] b) mesophase pitch derived open cell foam.

Carbon foam can be made from a variety of precursors. The precursor and the process conditions determine the properties of the foam; therefore carbon foam is a tailorable material [59]. It is denoted in the literature that, the size of the bubbles formed during the foaming can be changed by varying the operating conditions. Moreover other foam properties (density, porosity, strength and conductivity) can be affected. Higher density, increased compressive strength and a more interconnected open celled porous structure are obtained at higher pressures [88,89]. Pressure release time also affected the formation of porous structure. A more interconnected open-celled porous structure is formed for shorter pressure release times [89]. Solvents treatment ( tetrahydrofuran (THF), toluene, and xylene,) and additive additions (graphite powder, H₂O₂ treated isotropic Bulgarian pitch,

a

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Polymethylmethacrylate (PMMA) and Polystyrene (PS)) to the mesophase pitch decreased quality of foam, in terms of density, compressive strength, porosity and structure [88,90].

Carbon foam is an attractive alternative material to traditional materials in many applications due to its unique properties such as lightweight, high compressive strength, high or low thermal conductivity that depends on temperature while it is processing [56]. Carbon foams have nearly 90 % open pore structure. The density of carbon foam is ranging from 0.04 g/cm3 to 0.6 g/cm3. Carbon foam has high porous structure and uniform pore size distribution (average between 10 and 500 microns) [59]. Properties of various kinds of carbon foams from manufacturers are listed in Table 2.1 [91].

Table 2.1 : Properties of several kinds of carbon foams. Property Density (g/cm3) CTE (ppm/Co) Compressive Strength (MPa) Thermal Conductivity (W/mK) Ultramet’s RVC 0.042 1.15 – 1.65 0.763 0.085 ERG’s RVC N/a 1.2 – 1.8 0.28 – 0.48 N/a Touchstone 0.16 – 0.5 6.2 15.2 – 20.7 0.4 – 17.5 MER 0.016 – 0.62 N/a 1.7 - 7 0.05 - 210 PocoFoam 0.2 – 0.6 2 3.4 100 - 150

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Carbon foams can be used as a thermal insulator or conductor. Carbon foams produced below the temperature of 1200 oC have low thermal conductivity. In contrast, carbon foams produced above the temperature of 2500 oC have high thermal conductivity [79,92]. The carbon foam developed at Oak Ridge National Laboratory (ORNL) are four times more conductive than copper and six times more conductive than aluminum (approximately 1500W/m.K compared to 400 W/m.K for copper and 250 W/m.K for aluminum) [93-95].

Carbon foam shows highly ordered graphite properties when it is graphitized. It exhibits average interlayer spacing close to perfect graphite [12]. Carbon foams produced by Klett et al. has a 0.336 nm inter layer spacing and 203.3 nm coherent length (La) and 442 nm stacking height (Lc) [7].

2.8 Preparation and Characteristic of Mesophase Pitch Derived Graphitized Carbon Foam

2.8.1 An overview of carbon foam technology

Typical processes utilize a blowing technique is to produce carbon foam from the pitch precursor in which the pitch is melted and passed from a high-pressure region to a low-pressure region. Thermodynamically, this produces a "Flash" thereby causing the low molecular weight compounds in the pitch to vaporize (the pitch boils), resulting in a pitch foam. Then, the pitch foam must be oxidatively stabilized by heating in air (or oxygen) for many hours, thereby, cross-linking the structure and "setting" the pitch so it does not melt during carbonization. This is a time consuming step (up to 100 hours) and can be an expensive step depending on the particle size and equipment required. Without this oxidative stabilization step, the pitch may melt during further heat treatment [96,97].

The "set" or oxidized pitch is then carbonized in an inert atmosphere to temperatures as high as 1100ºC. Then, graphitization is performed at temperatures as high as 3000ºC to produce a high thermal conductivity graphitic structure (Figure 2.17).

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Load mold with resin

Saturate with” Blowing” Agent

Heat under high pressure to just above softenning point

Drop pressure “Flash”

Oxidatively stabilize

Carbonization,N2,1050 o

C Graphitize, >2400 oC

Figure 2.17 : Traditional “Blowing” technique [98].

Related process was developed and patented by the researchers from US Air Force Materials Laboratory. Carbon and graphite foams produced according to the related process can be used for many applications, including core material to replace aluminum in honeycomb panels, composite mandrels or tooling, sound insulation around engine cars and support structure for satellite antennas [98].

Other techniques utilize a polymeric precursor, such as phenolic, urethane, or blends of these with pitch. High pressure is applied and the sample heated. At the specified temperature, the pressure is released, thus causing the liquid to foam as volatile compounds are released. The polymeric precursor are cured and then carbonized without a stabilization step. However, these precursors produce a “glassy” or vitreous carbon which does not exhibit graphitic structure and, thus, has low thermal conductivity and low stiffness [97].

More recently, a breakthrough has come out about carbon foam, which would totally change the physical characteristics and application areas of the material. A process that does not require the “blowing” and “stabilization” steps was developed at Oak